setspeed

I'm arriving 0800hrs 13th September at Sydney airport with Malaysia Airlines.
I want to then fly on to Cairns. What sort of transfer time should I allow to get from Terminal 1 to (I assume) Terminal 2 or 3?

My choice of flight from Sydney to Cairns is 1100hrs or 1425hrs, both flights with Virgin. Is booking the 1100hrs one cutting it too fine?
I really, really DON'T want to hang around Sydney for 6 and a half hrs if I don't have to, following 25hrs travelling, and with another 4hr flight in front of me!

Thanks!